Isolated footing Design, Types & Reinforcement details

What Is an Isolated Footing?

An isolated footing is a type of foundation that is used to support a single, isolated column or post. The footing is typically circular or square in shape and is made of concrete. The footing is usually reinforced with a steel rebar to help improve its strength and load-bearing capacity. Isolated footings are typically used for structures that are not load-bearing, such as fences or light poles.

An isolated footing is one of the most used types of foundation to support single columns when they are arranged at a long distance. An isolated footing must be designed to avoid exceeding its bearing capacity and provide safety against overturning or sliding while preventing the ground from settling. The main concentration for an isolated footing is that it should have a bearing capacity at least twice as high as the expected gross floor load.

The following basic information needs to be known in the design of an isolated footing:

  1. The gross floor load is supported by the isolated footing, the load caused by partitions and any other loads that may affect the floor. The effect of any other dead load should be eliminated.
  2. Information about existing foundations and adjacent structures that may affect the isolated footing, such as underground utility lines, parking garage foundations, etc., should also be considered in order to avoid interference with its design way.

The gross floor load is determined from a study of building geometry and can be calculated based on a number of algorithms or tables depending on what is more convenient for the designer. The algorithms and tables are intended to reduce the amount of work needed to calculate the floor load. This is achievable because most calculations can be done easily based on geometry and/or materials information.

The calculation of the gross floor load is complicated by many factors including dead loads, live loads, types of materials used and distribution through the building.

Isolated Footing Design

Things to consider before the construction of Isolated footing:

  • Positioning of the footing on the ground
  • Area of footing. It can be calculated by considering factored loads.
  • Footing thickness. The amount of thickness that the footing has is expressed in millimetres.
  • Reinforcement details of the footing,with moment and shear forces are taken into consideration.
  • Check for shearing forces as well as the length of the formation.
  • Find the flexure critical section and the shear critical section.
  • Perform the necessary calculations to determine the bending moment as well as the shear forces in the critical section.
  • Examine the specifics of the reinforcement by considering bearing tensions, and making any necessary adjustments.
  • All of this is accomplished while taking into consideration the loads that will be placed on the footings, the safe bearing capacity (SBC) of the soil, the grade of the concrete, and the steel. The minimum cover to major reinforcement that must be present on the surface that is in touch with the earth’s face is 50 millimetres. The diameter of the primary reinforcement has to be at least 10 millimetres.

On all sides of the concrete bed, a 15 cm offset is normally provided. In the case of brick masonry columns, a 5 cm offset is likewise given in regular layers on all four sides.

Isolated Footing Reinforcement details

When it comes to the structural design of a footing, the footing reinforcement details are as important as the site analysis of the area where it is getting placed. The need for structural stability in the construction of the footing may be seen reflected in the details. Examples of appropriate reinforcement details include a cover-to-reinforcement ratio that is determined by environmental considerations in order to ensure the structure’s durability, minimum reinforcement required and bar diameters, and acceptable footing dimensioning. When creating designs, it is important to depict a solid foundation using both the plan and the elevation views. As an outcome of this, the following sections will discuss different components of the isolated footing reinforcement details.

The specifications of the isolated footing reinforcements are as follows:

  • Reinforcement concrete cover
  • The diameter of the bar and the minimum amount of reinforcement required
  • Distribution of reinforcement in isolated footing
  • Dowels are used to provide reinforcement.
  • Splice in the lap

Also read: Combined Footing Design, Types, Dimensions, Adv. & Disadvantages

If a footing is going to be in direct contact with the surface of the earth, then the minimum thickness of the main reinforcement in the footing should be 50 millimetres. On the other hand, the minimum thickness of the main reinforcement should be 40 millimetres for surfaces that are going to be exposed to the elements, such as surface levelling PCC. This information has been taken from the International standard  IS 456-200.

Bar Diameter and Minimum Reinforcement

A total of 0.12 percent of the cross-sectional area must be reinforced at the very least. A diameter of at least 10 millimetres is required for the main reinforcement.

Distribution of Footing Reinforcement

Reinforcement is spread out evenly throughout the whole width of a one-way RCC footing. This ensures that the footing will have the necessary strength. When a two-way reinforcement is used, the reinforcement is distributed uniformly throughout the whole width of the footing and extends in both directions. In the case of two-way rectangular footings, reinforcement is laid out along the length of the footing in such a way that it covers the whole width.

The reinforcement is spread over a short distance in the centre band, as calculated below. The remainder reinforcement is uniformly placed on both sides of the core band in the short direction.

Reinforcement distribution

Where y is the long side and x is the short side of the footing.

Reinforcement distribution in square isolated footing
Reinforcement distribution is rectangular isolated footing
Reinforcement distribution is rectangular isolated footing

Dowel Reinforcement

In order to strengthen the connection between the isolated footing and the structure’s top column, dowel reinforcement is used. Dowel reinforcement development requires the development length of dowel bars into the column and the isolated footing to be determined, and it must be illustrated in a clear and thorough manner on the design drawings. In addition, the development length of dowel bars into the column must be determined.

Dowel bars Reinforcement details

Lap Splice

The dowel lengths as well as the column reinforcement splice lengths must be plainly visible. To prevent the dowels in the footing failing to join and the lap splices between the dowels and the column bars failing, it is necessary to monitor the anchoring of both the flexural reinforcement lengths and the dowel reinforcement lengths.

Anchorage reinforcement
Section view of reinforcement details of isolated footing
Section view of reinforcement details of isolated footing
Plan view of reinforcement details of isolated footing (Typical Reinforcement details)
Plan view of reinforcement details of isolated footing (Typical Reinforcement details)

Application of Isolated footings

The types of projects where this style of footing is used include Well tanks, Staircases, Floor joints, Flooring deck for retaining walls, Flooring decks for buildings, Flooring deck for pavilions.

If an edge has to be cut in the footings and walls or other parapets are being reconstructed and need some strengthening or reinforcement, the stepped footings, sloped footings or the stepped spread footing would be the ideal type to be used for the same. Make sure the concrete is of good quality. Make sure the mortar joints are good.

Isolated footings are the most common kind of footing used for reinforced cement concrete columns. This is due to the fact that they are not only uncomplicated but also economical. A single footing serves as the foundation for a column that stands alone. The term “isolated footings” refers to footings that are provided individually for each column.

When this form of footing is required?

The columns are not close enough.
Footing loads are less.
Places where soil’s Safe bearing capacity (SBC) is high.

Advantages of Isolated footing

  • The cost of isolated foundations is quite low.
  • It is simple to build isolated footings.
  • Less earth excavation is required.
  • High-skilled labour are not required.

The use of isolated footings is an effective method for providing support for structures in challenging environments, such as on soft ground or in water, for example. They are easy to construct and may be utilised as support for a broad range of different kinds of structures. On softer terrain, such as the surface of a marsh, isolated footings may be utilised to support structures. They can also be used to support a structure in water.

Disadvantages of Isolated footings

  • The earth must be stable all the way around the structure’s foundation.
  • It may be extremely huge in size to deal with the heavy load.
  • This design is prone to the differential settlement because to the separate foundations, which might cause harm to the building.

Types of Isolated Footings

There are three types of Isolated footings:

  • Pad Isolated Foundation
  • Sloped Isolated Foundation
  • Stepped Isolated Foundation
Types of Isolated Footings
Types of Isolated Footings

What is a pad footing?

It is constructed one at a time under each column and often takes the shape of a square, rectangular, or circular pattern. The thickness of the flat isolated footing remains steady. It is included into the design such that bending moments and shear pressures at critical points are reduced. To improve the ultimate load-bearing capacity, it may be built from plain concrete or reinforced concrete.

Also read: Pile Foundations: Types of pile foundation, Design and Details

A pad footing is a type of foundation typically used for smaller structures such as houses. The pad footing is placed under each corner of the structure and is usually made of concrete. The size and shape of the pad footing is determined by the size and weight of the structure it is supporting. The pad footing must be large enough to distribute the weight of the structure evenly and must be deep enough to reach the soil below the frost line..

What is a stepped footing?

This kind of isolated footing was formerly widespread, but it has become less so in recent years. It’s often used in the construction of residential buildings. Stairs are formed by layering stepped footings on top of one another. Three concrete cross-sections are stacked on top of each other to create platforms.

What is a sloped footing?

Sloped or trapezoidal footings are carefully designed and constructed to maintain a 45-degree top slope on all sides. The sloped footing construction utilises less concrete and reinforcing than the isolated footing structure. As a consequence, less concrete and reinforcement is required.

Isolated footing benefits

There are several benefits to using an isolated footing, including:

• Reduced potential for damage: Since an isolated footing only supports a single column or support member, there is less potential for damage to nearby features if the footing were to shift or settle.

• Greater flexibility: An isolated footing can be placed in a wide variety of locations, including on sloped or uneven ground, making it a more versatile option than a continuous footing.

• Easier to construct: Isolated footings are typically easier to construct than continuous footings, since they require less excavation and concrete.

When is Isolated footing used

Isolated footing are typically used when a column or other vertical support member is located in close proximity to a property line, another building, or some other feature that prevents the placement of a traditional, continuous footing. In these cases, an isolated footing is the best way to provide support for the column while minimizing the potential for damage to nearby features.

Leave a Reply